Within the last decade there has been a substantial movement away from the use of older explosive compositions such as TNT, dynamite, and nitroglycerin as blasting agents for hard rock mining, excavation, and similar commercial purposes, in favor of prilled ammonium nitrate (AN) or mixtures of ammonium nitrate with other oxygen containing inorganic salts, plus various carbonaceous fuels and sensitizers (ANFO).
Such explosive formulations use relatively cheap components, can be produced or constituted "in situ", and are much safer compared with traditional explosives since the usual long term storage and transportation problems are substantially avoided.
Where a bore hole is wet and deep, however, the above-listed advantages of AN and ANFO formulations become less attractive since much material is easily desensitized and must be protected (a) by special packaging, (b) by the addition of expensive AN coatings, and (c) by use of thickeners or gelling agents, and the like. Moreover, ANFO has somewhat limited blasting energy due to its low bulk density.
Some of the above problems of AN and ANFO formulations can be avoided, at least in part, by use of special slurries having an increased bulk density, but such compositions are usually not moisture proof, and must continue to rely upon gassification or similar art-recognized density control means to retain an acceptable level of sensitivity. Pressure conditions at the bottom of deep bore holes, however, tend to compress gas voids causing temporary loss of sensitivity and detonation failure, leaving unexploded charges in areas where further drilling, mining or excavation may be required.
A substantial breakthrough with respect to moisture resistance is described in U.S. Pat. No. 3,161,551 of Egly, in which is water-resistant blasting agent is obtained having at least one solid prilled inorganic salt (e.g. ammonium nitrate "AN"), treated with a water-in-oil emulsion having a 50-70% AN solution and an art-recognized hydrocarbon fuel, tall oil, long chain fatty acids or derivatives thereof as the continuous external hydrophobic phase, plus a hydrophobic w/o emulsifier.
Egly's composition exhibits substantial resistance to water because of the fact that the water-in-oil emulsion component fills most of the natural voids in the solid prilled ammonium nitrate salt component, and water cannot easily force its way through the continuous external hydrophobic phase of the emulsion. Package charges of this type can be more readily placed at the bottom of a column of water than ANFO composites, because their composite density is substantially greater than 1.0. This combination of high density and solid oxidizer salt, however, results in a substantial decrease in detonation rate and overall energy efficiency, plus an unaccountably low sensitivity. Moreover, over a period of time, the presence of such solid salt, particularly solid ammonium nitrate, may even promote crystal growth in the aqueous phase of the emulsion which, in turn, leads to even further loss in sensitivity and energy efficiency.
Bluhm (U.S. Pat. No. 3,447,978), representing a further development in this area, describes an improved class of explosive compositions generally falling within the category of water-in-oil emulsion blasting agents. Such consist essentially of
(1) known water-in-oil emulsifiers; PA1 (2) a discontinuous ammonium nitrate aqueous phase (optionally supplemented by other water soluble oxidizer salts) within a continuous organic phase consisting of a carbonaceous fuel having a predetermined gas-retaining consistency at 70.degree. F.; and PA1 (3) a functionally important occluded gas such as air, or gas-entraining particles such as glass bubbles and the like, as a density control agent. PA1 (a) an internal discontinuous aqueous phase containing about 50-90 weight percent of a water soluble inorganic oxygen-supplying salt; and PA1 (b) an active amount of a continuous hydrophobic organic phase of a partial ester of a 2-12 carbon polyhydric alcohol esterified by a tall oil fatty acid; PA1 (a) an internal discontinuous aqueous phase of about 10-20 weight percent water or higher, based on emulsion and containing the above-indicated weight percent range of dissolved inorganic oxygen-supplying salt. Such salt is usually ammonium nitrate (AN) alone or in combination with one or more supplemental water soluble inorganic oxygen-supplying salt, preferably in a ratio by weight of about 3-4 parts AN to 1 part supplemental salt. PA1 (b) an effective amount of the partially esterified continuous hydrophobic organic phase which can vary substantially, depending upon the degree of negative Oxygen Balance the system can tolerate and the degree of sensitivity required; generally speaking, however, an active amount consists of about 5-10, and preferably 5-7 weight percent, based on total emulsion.
Bluhm's explosive compositions, while capable of avoiding some of the deep wet bore hole problems described above, also suffer from certain stability limitations and potential pollution problems because the proportion, by weight, of discontinuous aqueous phase to continuous organic phase must be high in order to even approximate a satisfactory Oxygen Balance. As a result, the oil films (continuous organic fuel and emulsifier phase) separating droplets of the discontinuous AN aqueous phase are very thin and easily broken by changes in ambient conditions, such as temperature and pressure variations. This, in turn, directly affects important explosive characteristics such as sensitivity.
Merely increasing the proportion of organic phase to aqueous phase, however, does not provide a satisfactory solution to Bluhm's problem since an increase in the organic phase inevitably results in a shift toward a negative Oxygen Balance, particularly if a substantial amount of packaging material such as polymeric film is required to retain or shape the charge.
The term Oxygen Balance (OB), as used above and hereafter, is conveniently defined percentage-wise by the formula EQU OB=(G.sub.o)/(G.sub.x).times.100
wherein G.sub.o is the number of grams of oxygen released (+) or oxygen required (-) in order to completely oxidize G.sub.x grams of the explosive formulations being tested.
For minimizing the formation of toxic or noxious by-product fumes, it is found that an overall Oxygen Balance of about -1% to +1% is required.
Water-in-oil type emulsifying agents, as separate components, are almost universally necessary in order to obtain stable formulations as exemplified in both Egly and Bluhm. Such may include fatty acid derivatives as listed, for instance, in U.S. Pat. Nos. 3,161,551, 3,447,978, 3,765,964, 4,110,134 and UK Pat. No. 1,306,546; nitrogen-containing surfactants such as ammonium salts as listed, for instance, in U.S. Pat. Nos. 4,026,738 and 4,141,767; oxazolines as listed in U.S. Pat. Nos. 4,216,040 and 4,322,258; hydroxyethyl-imidazolines as listed in U.S. Pat. No. 4,315,784; and taurates (e.g. U.S. Pat. No. 4,315,787).
Irrespective of one's choice of emulsifying agent, however, the need for ease in preparation and for storage stability under extreme ambient conditions has yet to be fully satisfied under Bluhm or other existing art teaching without also including a generous amount of a continuous organic phase. When provided, however, the resulting Oxygen Balance, as noted above, generally becomes substantially negative and dangerous toxic fumes are produced as a result.
It is an object of the present invention to obtain a class of explosive compositions which retain resistance to the deleterious effects of water and high pressure common to deep hole use.
It is a further object of the present invention to maximize both stability and explosive energy while maintaining acceptable booster sensitivity characteristics of a class of invert emulsion-containing explosive compositions.
It is a still further object to minimize the production of toxic and corrosive fumes produced as explosion by-products through use of an oxygen-balanced invert emulsion-containing blasting composition.